K. R. Anantharamaiah

STARBURST IN THE ULTRALUMINOUS GALAXY ARP 220: CONSTRAINTS FROM OBSERVATIONS OF RADIO RECOMBINATION LINES AND CONTINUUM

We present observations of radio recombination lines (RRL) from the starburst galaxy Arp 220 at 8.1 GHz (H92a) and 1.4 GHz (H167a and H165a) and at 84 GHz (H42a), 96 GHz (H40a) and 207 GHz (H31a) using the Very Large Array and the IRAM 30 m telescope, respectively. RRLs were detected at all the frequencies except 1.4 GHz, where a sensitive upper limit was obtained. We also present contin- uum —ux measurements at these frequencies as well as at 327 MHz made with the VLA. The continuum spectrum, which has a spectral index a D (0.6 between 5 and 10 GHz, shows a break near 1.5 (S l P la) GHz, a prominent turnover below 500 MHz, and a —atter spectral index above 50 GHz. We show that a model with three components of ionized gas with diUerent densities and area covering factors can consis- tently explain both RRL and continuum data. The total mass of ionized gas in the three components is 3.2 ) 107 requiring 3 ) 105 O5 stars with a total Lyman continuum production rate M _ , N Lyc D 1.3 ) 1055 photons s~1. The ratio of the expected to observed Bra and Brc —uxes implies a dust extinction mag. The derived Lyman continuum photon production rate implies a continuous star forma- A V D 45 tion rate (SFR) averaged over the lifetime of OB stars of D240 yr~1. The Lyman continuum photon M _ production rate of D3% associated with the high-density H II regions implies a similar SFR at recent epochs (t \ 105 yr). An alternative model of high-density gas, which cannot be excluded on the basis of the available data, predicts 10 times higher SFR at recent epochs. If con—rmed, this model implies that star formation in Arp 220 consists of multiple starbursts of very high SFR (few times 103 yr~1) and M _ short duration (D105 yr). The similarity of IR excess, in Arp 220 to values observed in L IR /L Lya D 24, starburst galaxies shows that most of the high luminosity of Arp 220 is due to the ongoing starburst rather than to a hidden active galactic nucleus (AGN). A comparison of the IR excesses in Arp 220, the Galaxy, and M33 indicates that the starburst in Arp 220 has an initial mass function that is similar to that in normal galaxies and has a duration longer than 107 yr. If there was no infall of gas during this period, then the star formation efficiency (SFE) in Arp 220 is D50%. The high SFR and SFE in Arp 220 is consistent with their known dependences on mass and density of gas in star-forming regions of normal galaxies. Subject headings: galaxies: individual (Arp 220) ¨ galaxies: nucleigalaxies: starburst ¨ radio continuum: galaxiesradio lines: galaxies

Low-frequency carbon recombination lines in the central regions of the galaxy

We have used the Parkes 64 m telescope to study low-frequency carbon recombination lines from the southern portion of the Galactic plane in a frequency band 4 MHz wide centered at 76.4 MHz. We have found a very large line-forming region that extends for approximately 40 in Galactic longitude from 1 = 340° to l = 20°. The region is several degrees wide in latitude. From the variation of radial velocity and line width with Galactic longitude we obtain kinematic distances to the line-forming region in the range 0.5-4 kpc. This places the line-forming region in the Sagittarius and/or Scutum arms. We also find the lines approximately tangent to the Scutum arm at 1 = 312 . By observing the C441α, C555β, and C635γ lines, which all occur in the analyzed band, we estimate the variation of line strength with principal quantum number. This variation appears to be similar to that of the well-observed Perseus arm absorption in the direction of Cas A and suggests that a model for the Perseus arm line-forming region may be at least partially applicable to the Sagittarius/Scutum arm regions. The most likely sites for the formation of these lines are the cold neutral H I concentrations in the interstellar medium. We have also searched the data for other lines and, in particular, we find no trace of hydrogen recombination lines.

Very Large Array H92α and H53α Radio Recombination Line Observations of M82

We present high angular resolution (06) observations made with the Very Large Array of the radio continuum at 8.3 and 43 GHz, as well as H92α and H53α radio recombination lines from the nearby (~3 Mpc) starburst galaxy M82. In the continuum we identify 58 sources at 8.3 GHz, of which 19 have no counterparts in catalogs published at other frequencies. At 43 GHz we identify 18 sources, unresolved at 06 resolution, of which five were unknown previously. The spatial distribution of the H92α line is inhomogeneous; we identify 27 features; about half of them are associated with continuum emission sources. Their sizes are typically in the range 2-10 pc. Although observed with poorer signal-to-noise ratio, the H53α line is detected. The line and continuum emission are modeled using a collection of H II regions at different distances from the nucleus. The observations can be interpreted assuming a single-density component, but equally well with two components if constraints originating from previous high-resolution continuum observations are used. The high-density component has a density of ~4 × 104 cm-3. However, the bulk of the ionization is in regions with densities that are typically a factor of 10 lower. The gas kinematics, using the H92α line, confirms the presence of steep velocity gradient (26 km s-1 arcsec-1) in the nuclear region, as previously reported, in particular from observations of the [Ne II] line at 12 μm. This gradient has about the same amplitude on both sides of the nucleus. Since this steep gradient is observed not only on the major axis but also at large distances along a band at P.A. ~ 150°, the interpretation in terms of x2 orbits elongated along the minor axis of the bar, which would be observed at an angle close to the inclination of the main disk, seems inadequate. The observed kinematics cannot be modeled using a simple model that consists of a set of circular orbits observed at different tilt angles. Ad hoc radial motions must be introduced to reproduce the pattern of the velocity field. Different families of orbits are indicated since we detect a signature in the kinematics at the transition between the two plateaus observed in the NIR light distribution. These H92α data also reveal the base of the outflow where the injection toward the halo on the northern side occurs. The outflow has a major effect on the observed kinematics, present even in the disk at distances close to the nucleus. The kinematic pattern suggests a connection between the gas flowing in the plane of M82 toward the center; this behavior most likely is due to the presence of a bar and the outflow out of the plane.

HIGH RYDBERG STATE CARBON RECOMBINATION LINES TOWARD CASSIOPEIA A : PHYSICAL CONDITIONS AND A NEW CLASS OF MODELS

We present models of physical conditions in Perseus arm clouds observed in the direction of Cas A. Theoretical predictions of carbon radio recombination line intensities are compared with observations spanning the frequency range 14 to 775 MHz. Best-fitting model parameters are then combined with the results of lambda 21 cm H I absorption line observations, and low-frequency hydrogen recombination line observations, to evaluate the thermal and pressure balance in these clouds. A critical reexamination of the available recombination line data shows that the lowest frequency carbon line observations have underestimated the integrated optical depths in the line by up to a factor of 3. This is due to the removal of large Lorentzian wings from the pressure broadened line profiles during baseline subtraction. Models where some or all of the recombination line optical depth originates in molecular clouds do not give satisfactory results. Models based on standard calculations of recombination line intensites (Salem & Brocklehurst 1979; Walmsley & Watson 1982) have high pressures and are out of thermal balance. We modified the theoretical calculation of line intensities, described by Salem & Brocklehurst (1979) and Walmsley & Watson (1982), by imposing a different boundary condition at large principal quantum number n. Instead of assuming an infinite number of levels populated according to thermodynamic equilibrium, we impose a cutoff in level populations above a critical principal quantum number n cr , as suggestd by Gulyaev & Nefedov (1989). We followed the occupation probability formalism presented by Hummer & Mihalas (1988) to calculate n cr . Using these new line intensity calculations, we find that, with the exception of the recombination line width, all of the Cas A recombination line and lambda 21 cm H I absorption line data can be attributed to a region where the physical conditions are typical of the cold neutral medium of the interstellar medium. We show a model in which this region is in thermal balance at a temperature near 36 K and has a reasonable interstellar pressure. In this model, the photoionization of polycyclic aromatic hydrogen molecules is the dominant mechanism for heating the region.

Very Large Array Detection of Radio Recombination Lines from the Radio Nucleus of NGC 253: Ionization by a Weak Active Galactic Nucleus, an Obscured Super Star Cluster, or a Compact Supernova Remnant?

We have imaged the H92α and H75α radio recombination line (RRL) emissions from the starburst galaxy NGC 253 with a resolution of ∼4 pc. The peak of the RRL emission at both frequencies coincides with the unresolved radio nucleus. Both lines observed towards the nucleus are extremely wide, with FWHM of ∼200 km s. Modeling the RRL and radio continuum data for the radio nucleus shows that the lines arise in gas whose density is ∼10 cm and mass is few thousand M⊙, which requires an ionizing flux of 6–20×10 photons s. We consider a SNR expanding in a dense medium, a star cluster and also an AGN as potential ionizing sources. Based on dynamical arguments, we rule out an SNR as a viable ionizing source. A star cluster model was considered and the dynamics of the ionized gas in a stellar-wind driven structure was investigated. Such a model is consistent with the properties of the ionized gas only for a cluster younger than ∼10 years. The existence of such a young cluster at the nucleus seems improbable. The third model assumes the ionizing source to be an AGN at the nucleus. In this model, it was shown that the observed X-ray flux is too weak to account for the required ionizing photon flux. However, the ionization requirement can be explained if the accretion disk is assumed to have a Big Blue Bump in its spectrum. Hence, we favor an AGN at the nucleus as the source responsible for ionizing the observed RRLs. A hybrid model consisting of a inner ADAF disk and an outer thin disk is suggested, which could explain the radio, UV and the X-ray luminosities of the nucleus. Subject headings: galaxies: individual (NGC 253) — galaxies: ISM — galaxies: nuclei — galaxies: starburst — radio lines: galaxiesWe have imaged the H92alpha and H75alpha radio recombination line (RRL) emissions from the starburst galaxy NGC 253 with a resolution of similar to4 pc. The peak of the RRL emission at both frequencies coincides with the unresolved radio nucleus. Both lines observed toward the nucleus are extremely wide, with FWHMs of similar to200 km s(-1). Modeling the RRL and radio continuum data for the radio nucleus shows that the lines arise in gas whose density is similar to10(4) cm(-3) and mass is a few thousand M., which requires an ionizing flux of (6-20) x 10(51) photons s(-1). We consider a supernova remnant (SNR) expanding in a dense medium, a star cluster, and also an active galactic nucleus (AGN) as potential ionizing sources. Based on dynamical arguments, we rule out an SNR as a viable ionizing source. A star cluster model is considered, and the dynamics of the ionized gas in a stellar-wind driven structure are investigated. Such a model is only consistent with the properties of the ionized gas for a cluster younger than similar to10(5) yr. The existence of such a young cluster at the nucleus seems improbable. The third model assumes the ionizing source to be an AGN at the nucleus. In this model, it is shown that the observed X-ray flux is too weak to account for the required ionizing photon flux. However, the ionization requirement can be explained if the accretion disk is assumed to have a big blue bump in its spectrum. Hence, we favor an AGN at the nucleus as the source responsible for ionizing the observed RRLs. A hybrid model consisting of an inner advection-dominated accretion flow disk and an outer thin disk is suggested, which could explain the radio, UV, and X-ray luminosities of the nucleus.

Very Large Array Observations of the H92α Line from NGC 5253 and Henize 2-10: Ionized Gas around Super Star Clusters

We have detected the H92α radio recombination line from two dwarf starburst galaxies, NGC 5253 and He 2-10, using the Very Large Array. Both the line data as well as the radio continuum data are used to model the properties of the ionized gas in the centers of these galaxies. We consider a multidensity model for radio recombination lines and show why previous models, which were based on the assumption of gas at a single density, are valid in many situations. The models show that the ionized gas has a density of ~10 4 cm -3 in both galaxies, with an effective size of 2-10 pc and a total mass of ~10 4 M ⊙ . The derived production rate of Lyman continuum photons is ~2.5 × 10 52 s -1 in both the galaxies, and the corresponding mass of stars (assuming a Salpeter initial mass function) is ~10 5 M ⊙ . The implied stellar density shows that the observed radio recombination lines arise from ionized gas around super star clusters (SSCs) in both galaxies (these SSCs have been recently detected through their radio continuum emission). The existence of ~104 M ⊙ of ionized gas within a few parsecs of an SSC places strict constraints on dynamical models. Using simple arguments, the parameter space for a few possible models are derived. The well-known radio-far-infrared correlation also holds for NGC 5253, although the radio emission from this galaxy is almost completely thermal. It is shown that NGC 5253 is strong evidence that the component of far-infrared emission from warm dust is correlated separately with the component of radio emission from thermal bremsstrahlung.

Low density ionized gas in the inner galaxy - Interpretation of recombination line observations at 325 MHz

The recent survey of H 272α recombination line (324.99 MHz) in the direction of 34 Hn regions, 12 SNRs and 6 regions of continuum minimum (‘blank’ regions) in the galactic plane is used to derive the properties of diffuse ionized gas in the inner Galaxy.The intensity of radio recombination lines at high frequencies is dominated by spontaneous emission in high-density gas and that at low frequencies (325 MHz) by stimulated emission in low-density gas. We have used this property to obtain the electron density in the gas in the direction of blank regions and SNRs, by combining the H 272 α measurements (preceeding paper) with the published data at higher frequencies. Further, we have imposed constraints on the electron temperature and pathlength through this gas using the observed high-frequency continuum emission, average interstellar electron density and geometry of the line-emitting regions. The derived properties of the gas are (i) electron density 0.5–6 cm-3, (ii) electron temperature 3000–8000 K and (iii) emission measures 500–3000 pc cm-6 The corresponding pathlengths are 50–200 pc.As the derived sizes of the low-density regions are small compared to the pathlength through the Galaxy, the low-frequency recombination lines cannot be considered as coming from a widely distributed component of the interstellar medium.The Hn regions studied in the above survey cannot themselves produce the H 272α lines detected towards them because of pressure broadening, optical depth, and beam dilution. However, the agreement in velocity of these lines with those seen at higher frequencies suggests that the low-frequency recombination lines arise in low-density envelopes of the Hn regions. Assuming that the temperature of the envelopes are similar to those of the cores and invoking geometrical considerations we find that these envelopes should have electron densities in the range 1–10 cm-3 and linear sizes of 30–300 pc in order to produce the observed H 272α lines.

HYDROGEN RECOMBINATION LINES NEAR 327 MHz. III. PHYSICAL PROPERTIES AND ORIGIN OF THE LOW-DENSITY IONIZED GAS IN THE INNER GALAXY

We present constraints on the physical properties of the ionized gas responsible for hydrogen radio recombination lines (RRLs) near 327 MHz detected in a recent Galactic plane survey made with the Ooty Radio Telescope. To obtain these constraints, we combined the data at 327 MHz with previously published RRL observations near 1.4 GHz. The density of the ionized gas is well constrained and is in the range of 1 to 10 cm-3. The data implies upper limits to the temperature and size of the line emitting regions of ~12,000 K and ~500 pc, respectively. Assuming an electron temperature of 7000 K, the derived path lengths of the line emitting region are in the range of 20 to 200 pc. The derived properties of the ionized gas responsible for the RRL emission near 327 MHz suggest that most of the [N II] 205 μm emission and a considerable fraction of the [C II] 158 μm emission observed in the Galactic plane by the COBE satellite could also originate in the same gas. The Hα emission from these ionized gases is mostly undetected in the existing Hα surveys because of large interstellar extinction. About 50% of the free-free absorption of the Galactic nonthermal radiation observed at frequencies less than 100 MHz can be accounted for by the same ionized gas. We also discuss the origin of this low-density ionized gas in the inner Galaxy. The derived low line-of-sight filling factor (<1%) for this ionized gas indicates that it does not form a pervasive medium. On the basis of the similarity of the distribution of this gas in the Galactic disk with that of the star-forming regions and the range of derived physical properties, we support the earlier suggestion that the low-frequency RRL emission originates from low-density ionized gas, which forms envelopes of normal H II regions.

Very Large Array H53α and H92α Line Observations of the Central Region of NGC 253

We present new VLA observations toward NGC 253 of the recombination line H53α (43 GHz) at an angular resolution of 15 × 10. The free-free emission at 43 GHz is estimated to be ~140 mJy, implying a star formation rate of 2 M☉ yr-1 in the nuclear region of this starburst galaxy. A reanalysis is made for previously reported H92α observations carried out with angular resolution of 15 × 10 and 036 × 021. Based on the line and continuum emission models used for the 15 × 10 angular resolution observations, the RRLs H53α and H92α are tracers of the high-density (~105 cm-3) and low-density (~103 cm-3) thermally ionized gas components in NGC 253, respectively. The velocity fields observed in the H53α and H92α lines (15 × 10) are consistent. The velocity gradient in the central ~18 pc of the NE component, as observed in both the H53α and H92α lines, is in the opposite direction to the velocity gradient determined from the CO observations. The enclosed virial mass, as deduced from the H53α velocity gradient over the NE component, is ~5 × 106 M☉ in the central ~18 pc region. The H92α line observations at high angular resolution (036 × 021) reveal a larger velocity gradient, along a P.A. ~ -45° on the NE component, of ~110 km s-1 arcsec-1. The dynamical mass estimated using the high angular resolution H92α data (~7 × 106 M☉) supports the existence of an accreted massive object in the nuclear region of NGC 253.

On the origin of the galactic ridge recombination lines

Radio recombination lines are known to be observable at positions along the galactic ridge which are free of discrete continuum sources. Based on the results of a recent survey of H272α lines it is shown that most of the observed galactic ridge recombination lines can be explained as emission from outer low-density envelopes of normal Hn regions. The distribution of low-density ionized gas and discrete HII regions as a function of the distance from the galactic centre is also derived.